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Xanthophyll
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{{Short description|Chemical compounds subclass}} {{about|Xanthophylls|the compound xanthophyll|lutein}} [[Image:Raw egg.jpg|thumb|The characteristic color of [[egg yolk]] results from the presence of a xanthophyll pigment typical in color of lutein or zeaxanthin of the xanthophylls, a division of the carotenoids group.]] '''Xanthophylls''' (originally '''phylloxanthins''') are yellow [[pigment]]s that occur widely in nature and form one of two major divisions of the [[carotenoid]] group; the other division is formed by the [[carotene]]s. The name is from Greek: {{Transliteration|el|xanthos}} ({{lang|el|ξανθός}}), meaning "yellow",<ref name=LSJ1>{{LSJ|canqo/s|ξανθός|ref}}</ref> and {{Transliteration|el|phyllon}} ({{lang|el|φύλλον}}), meaning "leaf"),<ref name=LSJ2>{{LSJ|fu/llon|φύλλον|ref}}</ref> due to their formation of the yellow band seen in early [[history of chromatography|chromatography]] of [[leaf]] pigments. ==Molecular structure== [[Image:Cryptoxanthin.svg|thumb|right|400px|The chemical structure of [[cryptoxanthin]]. Xanthophylls typically present oxygen as a [[hydroxyl group]].]] [[File:Chromatography of chlorophyll - Step 7.jpg|right|thumb|130px|Thin layer chromatography is used to separate components of a plant extract, illustrating the experiment with plant pigments that gave chromatography its name. Plant xanthophylls form the bright yellow band next to the green.]] As both are carotenoids, xanthophylls and carotenes are similar in structure, but xanthophylls contain [[oxygen]] atoms while carotenes are ''purely [[hydrocarbon]]s'', which do not contain oxygen. Their content of oxygen causes xanthophylls to be more [[polar molecule|polar]] (in molecular structure) than carotenes, and causes their separation from carotenes in many types of [[chromatography]]. (Carotenes are usually more orange in color than xanthophylls.) Xanthophylls present their oxygen either as [[hydroxyl group]]s and/or as hydrogen atoms substituted by oxygen atoms when acting as a bridge to form [[epoxide]]s. ==Occurrence== Like other carotenoids, xanthophylls are found in highest quantity in the [[leaves]] of most green [[plants]], where they act to modulate light energy and perhaps serve as a [[non-photochemical quenching]] agent to deal with triplet [[chlorophyll]] (an excited form of chlorophyll),{{citation needed|date=June 2012}} which is overproduced at high light levels in photosynthesis. The xanthophylls found in the bodies of animals including humans, and in dietary animal products, are ultimately derived from plant sources in the diet. For example, the yellow color of chicken [[Egg (biology)|egg]] [[yolk]]s, fat, and skin comes from ingested xanthophylls—primarily [[lutein]], which is added to chicken feed for this purpose. The yellow color of the [[macula lutea]] (literally, ''yellow spot'') in the [[retina]] of the human eye results from the presence of [[lutein]] and [[zeaxanthin]]. Again, both these specific xanthophylls require a source in the human diet to be present in the human eye. They protect the eye from ionizing light (blue and ultraviolet light), which they absorb; but xanthophylls do not function in the mechanism of sight itself as they cannot be converted to [[retinal]] (also called retinaldehyde or [[vitamin A]] aldehyde). Their physical arrangement in the macula lutea is believed to be the cause of [[Haidinger's brush#Physiological causes|Haidinger's brush]], an [[entoptic phenomenon]] that enables perception of [[polarization (waves)|polarizing]] light. ==Example compounds== The group of xanthophylls includes (among many other compounds) [[lutein]], [[zeaxanthin]], [[neoxanthin]], [[violaxanthin]], [[flavoxanthin]], and α- and β-[[cryptoxanthin]]. The latter compound is the only known xanthophyll to contain a beta-ionone ring, and thus β-[[cryptoxanthin]] is the only xanthophyll that is known to possess pro-vitamin A activity for mammals. Even then, it is a vitamin only for plant-eating mammals that possess the enzyme to make retinal from carotenoids that contain beta-ionone (some carnivores lack this enzyme). In species other than mammals, certain xanthophylls may be converted to hydroxylated retinal-analogues that function directly in vision. For example, with the exception of certain flies, most insects use the xanthophyll derived R-isomer of 3-hydroxyretinal for visual activities, which means that β-[[cryptoxanthin]] and other xanthophylls (such as lutein and zeaxanthin) may function as forms of visual "vitamin A" for them, while carotenes (such as beta carotene) do not. ==Xanthophyll cycle== [[File:Violaxanthin cycle.png|thumb|400px|The xanthophyll cycle]] The xanthophyll cycle involves the enzymatic removal of epoxy groups from xanthophylls (e.g. [[violaxanthin]], [[antheraxanthin]], [[diadinoxanthin]]) to create so-called de-epoxidised xanthophylls (e.g. [[diatoxanthin]], [[zeaxanthin]]). These enzymatic cycles were found to play a key role in stimulating energy dissipation within light-harvesting antenna proteins by [[non-photochemical quenching]]- a mechanism to reduce the amount of energy that reaches the photosynthetic reaction centers. Non-photochemical quenching is one of the main ways of protecting against [[photoinhibition]].<ref>Falkowski, P. G. & J. A. Raven, 1997, Aquatic photosynthesis. Blackwell Science, 375 pp</ref> In higher plants, there are three carotenoid pigments that are active in the xanthophyll cycle: violaxanthin, antheraxanthin, and zeaxanthin. During light stress, violaxanthin is converted, i.e. reduced, to zeaxanthin via the intermediate antheraxanthin, which plays a direct photoprotective role acting as a lipid-protective [[anti-oxidant]] and by stimulating non-photochemical quenching within light-harvesting proteins. This conversion of violaxanthin to zeaxanthin is done by the enzyme violaxanthin de-epoxidase ([[Enzyme Commission number|EC]] [https://www.brenda-enzymes.org/enzyme.php?ecno=1.23.5.1 1.23.5.1]), while the reverse reaction, i.e. oxidation, is performed by zeaxanthin epoxidase ([[Enzyme Commission number|EC]] [https://www.brenda-enzymes.org/enzyme.php?ecno=1.14.15.21 1.14.15.21]).<ref>Taiz, Lincoln and Eduardo Zeiger. 2006. ''Plant Physiology''. Sunderland, MA: Sinauer Associates, Inc. Publishers, Fourth edition, 764 pp</ref> In [[diatoms]] and [[dinoflagellates]], the xanthophyll cycle consists of the pigment [[diadinoxanthin]], which is transformed into [[diatoxanthin]] (diatoms) or [[dinoxanthin]] (dinoflagellates) under high-light conditions.<ref>Jeffrey, S. W. & M. Vesk, 1997. Introduction to marine phytoplankton and their pigment signatures. In Jeffrey, S. W., R. F. C. Mantoura & S. W. Wright (eds.), Phytoplankton pigments in oceanography, pp 37-84. – UNESCO Publishing, Paris. </ref> Wright et al. (Feb 2011) found that, "The increase in zeaxanthin appears to surpass the decrease in violaxanthin in spinach" and commented that the discrepancy could be explained by "a synthesis of zeaxanthin from beta-carotene", however they noted further study is required to explore this hypothesis.<ref>{{cite journal|last=Wright|title=The interrelationship between the lower oxygen limit, chlorophyll fluorescence and the xanthophyll cycle in plants|journal=Photosynthesis Research|volume=107|issue=3|pages=223–235|display-authors=etal|doi=10.1007/s11120-011-9621-9|pmid=21290261|year=2011|s2cid=8454497}}</ref> ==Food sources== Xanthophylls are found in all young leaves and in [[etiolation|etiolated]] leaves. Examples of other rich sources include [[papaya]], [[peach]]es, [[prune]]s, and squash, which contain lutein diesters.<ref>[http://jn.nutrition.org/content/132/3/531S.full Factors That Influence the Bioavailablity of Xanthophylls, Susan Zaripheh, John W. Erdman Jr.]</ref><ref>{{Cite web |url=http://www.botgard.ucla.edu/html/botanytextbooks/generalbotany/shootfeatures/generalstructure/leafcolor/xanthophylls.html |title=UCLA College of Life Sciences, General Botany:Leaf Color: Xanthophylls |access-date=2014-08-03 |archive-date=2016-08-25 |archive-url=https://web.archive.org/web/20160825190141/http://www.botgard.ucla.edu/html/botanytextbooks/generalbotany/shootfeatures/generalstructure/leafcolor/xanthophylls.html |url-status=dead }}</ref><ref>{{cite web|url=http://www.livestrong.com/article/260693-foods-that-contain-zeaxanthin/ |author=Michele Turcotte, MS, RD |title=Foods That Contain Zeaxanthin |date=January 7, 2016 |archive-date=September 16, 2017 |archive-url=https://web.archive.org/web/20170916061820/http://www.livestrong.com/article/260693-foods-that-contain-zeaxanthin/}}</ref> [[Kale]] contains about 18 mg lutein and zeaxanthin per 100g, [[spinach]] about 11 mg/100g, [[parsley]] about 6 mg/100g, [[peas]] about 3 mg/110g, [[Cucurbita|squash]] about 2 mg/100g, and [[pistachios]] about 1 mg/100g.<ref>{{cite journal |last1=Eisenhauer |first1=Bronwyn |last2=Natoli |first2=Sharon |last3=Liew |first3=Gerald |last4=Flood |first4=Victoria M. |title=Lutein and Zeaxanthin—Food Sources, Bioavailability and Dietary Variety in Age-Related Macular Degeneration Protection |journal=Nutrients |date=9 February 2017 |volume=9 |issue=2 |pages=120 |pmc=5331551 |pmid=28208784 |doi=10.3390/nu9020120 |doi-access=free }}</ref> ==References== <references/> *Demmig-Adams, B & W. W. Adams, 2006. Photoprotection in an ecological context: the remarkable complexity of thermal energy dissipation, New Phytologist, 172: 11–21. ==External links== * {{MeshName|Xanthophylls}} {{Plant Pigments}} {{Carotenoids}} [[Category:Carotenoids]] [[Category:Fatty alcohols]]
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